This invention relates generally to forming magnetoresistive field sensors. More particularly, this invention relates to magnetic recording systems.
Sensors for detecting and measuring magnetic fields find many scientific and industrial applications. For example, a magnetic recording head typically includes a sensing element that senses a magnetic flux emanating from a recording medium. The magnetic field changes some physical property of the sensing element in a manner that depends on the magnitude and direction of the magnetic field. A sensing element that changes its electrical resistivity in response to a magnetic field is usually referred to as a magnetoresistive field sensor. Prior magnetoresistive field sensors typically include one or more ferromagnetic elements whose resistivity changes in response to magnetic flux. Prior magnetoresistive field sensors include anisotropic magnetoresistive (AMR) sensors and giant magnetoresistive. (GMR) sensors, in which a sense current flows along, or perpendicular to, planes of the ferromagnetic elements. Prior magnetoresistive field sensors also include magnetoresistive tunnel junction (MTJ) sensors, in which a sense current flows perpendicular to the planes of the ferromagnetic elements through a dielectric barrier. Resistance of a magnetoresistive field sensor varies as the square of the cosine of the angle between the magnetization in the sensor and the direction of sense current. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the sensor, which in turn causes a change in resistance in the sensor and a corresponding change in the sense current or voltage.
Increasing areal density of magnetic storage media requires that the magnetic recording and reading heads be able to operate at ever-decreasing track widths (TW). Both the write element and the magnetic readback sensor of the recording head must be made smaller in order to achieve narrower data tracks. The width of the recorded track is determined by, among other parameters, the width of the write pole of the write head and the flying height of the write head. The size and geometry of the shields and leads also play a role in determining achievable track width for a given recording head design.
In order to take advantage of the narrower write track width, it is imperative that the read track width of the readback element or read head be reduced as well. At present, magnetoresistive (MR) heads are typically made by photolithographically defining the sensor element from a continuous multilayer thin film. The sensor, which is frequently rectangular in shape, is often defined in two steps, one photolithographic step to define the TW dimension, and one lapping step to define the so-called xe2x80x9cstripe heightxe2x80x9d (SH) dimension. Unfortunately, due to practical limitations of the lithographic method, such as the diffraction limit of light, it is not easy in a manufacturing environment to produce read heads much narrower than about 200 nm. Meanwhile, MR head technology is already pushing present photolithographic techniques to their limits and these present methods will not be able to accommodate future generations of MR heads. For example, in current commercial products, the sensor TW, which is defined by optical lithography and ion beam milling, is typically less than 1 xcexcm. It is envisaged that in order to make heads suitable for recording densities of 100 Gbits/in2, the sensor TW will need to be around 0.13 xcexcm, but current lithography is wavelength-limited to around 0.2 xcexcm.
An associated problem that arises from the current processing method is poor shape definition, which leads to a xe2x80x9ctailxe2x80x9d on each side of the sensor. The tails are a result of the ion beam milling process commonly used to define TW. The milling is performed with the ion beam at an angle to the wafer in an effort to minimize the redeposition of magnetic material at the mask edges, which would have a deleterious effect on the sensor performance. However, ion milling at an angle creates a shadow near the mask edges, within which the milling is less efficient, resulting in tails on the sensor structure. The beam divergence from the ion mill also contributes to the tails. The presence of the tails degrades the magnetic performance of the sensor. Further, the tails may vary in dimension and form across the wafer, resulting in sensor-tosensor variation in performance. FIG. 1 illustrates a cross-sectional schematic diagram of a contiguous junction design MR sensor 100. MR sensor 100 includes a first magnetic shield 102, and a first insulating gap 104 disposed on the shield 102. The sensing element 106 including tails 108 is disposed on the gap 104. Following the milling process, the top of the multilayer sensing element 106 will have a width determined by the resist mask used. However, the all-important sense layer, which is located further down in the multilayer stack that forms sensing element 106, will inevitably have a larger and possibly not well-controlled width. This problem is predicted to become increasingly important as the TW decreases and the tails become proportionally larger relative to the sensor dimensions.
Once the sensing element 106 is formed using optical lithography and milling, it is usual to deposit a ferromagnetic layer, called xe2x80x9chard biasxe2x80x9d layer 110, with substantial magnetic coercivity (Hc) on each side of the sensing element 106 to stabilize the magnetization at each side of the sensing element, thereby improving sensor performance. However, the tails 108 on each side of the sensing element 106 make deposition of a uniform hard bias layer 110 difficult, and the hard bias layer 110 becomes very thin near the top surface of the sensing element 106 and/or does not closely abut the sensing element 106, leading to poor sensor performance.
MR sensor 100 further includes leads 112 adjacent to hard bias layers 110 to conduct the sense current to the sensing element 106 when reading data stored on a magnetic recording medium, a second gap 114 and a second shield 115 to protect the sensing element 106.
A U.S. patent application entitled xe2x80x9cTrack Width Control of Readback Elementxe2x80x9d field Jun. 30, 1999, to Patrick C. Arnett et al. discloses a method for reducing the track width of readback elements by implantation of ions. The ion implantation reduces the magnetoresistance of the selected portions of the readback elements. The ion implantation of Arnett et al. is performed by a focused ion beam (FIB) technique. However, FIB processing is slow, since each element is processed in series, which is not desirable for mass manufacture of magnetic sensors. Furthermore, electrostatic discharge (ESD) damage can occur during the application of the FIB to the sensor element, and therefore grounding during processing and low ion currents will be required to minimize this risk. In addition, the FIB processing of Arnett et al. is performed from the air-bearing surface (ABS). The layers that make up the sensor typically run perpendicular to the ABS and have stripe heights about an order of magnitude or more greater than the sensor film thickness. Consequently the ions must penetrate to a greater depth than the sensor film thickness in order to define the magnetically sensitive xe2x80x9ctip portionxe2x80x9d. A large depth requirement demands high ion energies (incidentally, well beyond the range of standard FIB machines). The increased ion energy will cause an increase in the lateral straggle of the ions in the sensor material, and will widen the transition region between the tip portion and the neighboring xe2x80x9cmagnetically deactivatedxe2x80x9d region, presumably degrading the performance of the sensor. In order to conduct xe2x80x9cimplantationxe2x80x9d amounting to a typical few atomic percent of the critical layers, this technique requires extremely large ion doses with long processing times, resulting in problems with heat dissipation and surface sputtering. Furthermore, this technique teaches an implantation based on a geometry which is quite unlike that used in recording heads or other MR sensors.
An article entitled xe2x80x9cPatterning Ferromagnetism in Ni80Fe20 Films via 30 keV Ga+ Ion Irradiationxe2x80x9d submitted to Applied Physics Letters on Mar. 30, 2000 by W. M. Kaminsky et al. discloses a method to degrade and even destroy the ferromagnetism of a GMR multilayer system, such as Ni80Fe20/Cu/Ni80Fe20/Ni80Cr20, by exposing this GMR multilayer system to homogeneous 30 keV Ga+ implantation. Ga+ implantation destroys all appearances of ferromagnetism at room temperature. The degradation of ferromagnetism occurs primarily because of ion implantation. Kaminsky et al. describe FIB irradiation of a single layer film to fashion a laterally patterned multilayer system. Such an approach would work for patterning the film from the ABS level to produce a read-back sensor. However, this is impractical for mass production of magnetic sensors for the reasons discussed above. Additionally, the lateral scattering of the implanted ions in the material is too great to produce implanted regions sufficiently narrow, and with sufficiently perfect interfaces, to allow a magnetoresistive sensor to be produced which would produce signals competitive with those from thin film sensors.
U.S. Pat. No. 5,079,662 issued on Jan. 7, 1992 to Kawakami et al. discloses a compound magnetic head in which the read element is sandwiched between the poles of the write gap. This patent has mentioned the ion implantation into selected areas of recording heads. However, the ion implantation is performed to increase the coercive field in those areas.
There is a need, therefore, for a MR recording head having improved definition of patterned magnetic sensors and a method of fabricating same.
Accordingly, it is a primary object of the present invention to provide a MR sensor with improved shape definition.
It is a further object of the invention to provide a MR sensor with controlled track width.
It is a further object of the invention to provide a MR sensor with well-controlled biasing for magnetic stabilization.
It is an additional object of the invention to provide a method for fabricating such a MR sensor.
These objects and advantages are attained by MR sensors with small track widths defined using ion irradiation and/or implantation at the wafer level.
According to a first embodiment of the present invention, a MR structure has a sensor defined by ion irradiation and/or implantation through a mask introduced between the film and the ion source at the wafer level. The unmasked portions of the MR structure are irradiated or implanted with ions, which reduces the magnetoresistance of the unmasked portions. However, materials of the unmasked ion-treated portions are still electrically conductive, which may be used as the lead, or part of the lead structure. Irradiation, as used herein, is distinguished from implantation in that ions irradiating a layer of material have sufficient energy to pass through the layer without being embedded in the layer. Furthermore, the irradiating ions have sufficient energy that they pass through the layer without significant sputtering or milling of the layer.
The mask may be a photolithographic resist mask located in contact with the surface of the magnetoresistive (MR) structure to cover selected portions of the MR structure and it is sufficiently thick to stop the ions incident on those regions, preventing them from reaching the MR structure. The unmasked portions are exposed to ion beams for patterning the sensor and reducing the magnetoresistance of the unmasked portions while leaving the masked portions, which define the track widths, magnetoresistive. Alternatively, an electron beam resist mask may be used. A designed TW may be achieved depending on the size of the resist mask used in irradiation and/or implantation process. Fabrication of a MR sensor using a resist mask allows for track width as small as 5 nm.
As an alternative to a resist mask, a stencil mask may be suspended above the surface of a MR structure during ion irradiation and/or implantation. The stencil mask may be produced using photolithography, electron-beam lithography, or other appropriate techniques. By choosing suitable ions and energies to minimize sputtering, the stencil mask may be used repeatedly, thus the cost of producing the mask is of small importance to manufacturing.
Alternatively, the sensor of a MR structure may be defined by ion irradiation and/or implantation using a projection ion beam system. In the projection ion beam system, a collimated ion beam is projected through a mask, which is disposed between the MR structure and an ion source. The ion beam is focused by beam optics after passing through the mask. A design TW may be achieved depending on the distance between the mask and the surface of the MR structure, the properties of the beam optics, and the size of features in the mask.
According to a second embodiment of the present invention, the magnetization of defined sensors of the types as described in the first embodiment may be stabilized by using a hard bias layer adjacent to the sensor. Alternatively, the magnetization of the defined sensor may be stabilized by using an anti-parallel (AP) pinning layer or an in-stack anti-ferromagnetic (AF) layer.
The methods of using ion implantation/irradiation for reducing the magnetoresistance and magnetic moment of the unmasked portions described in the first embodiment may be used for AMR, GMR and MTJ sensors. The magnetoresistance of a typical NiMn-based multilayer MR structure falls to around 3% of the as-grown value with an ion dose of 1016 ions/cm2. However, although the ion irradiation reduces the magnetoresistance of the unmasked portions, the magnetic moment of those portions is not substantially altered. Ion implantation with suitable species reduces the magnetic moment and the magnetoresistance of the unmasked portions to zero. The magnetoresistance of typical spin valve samples drops rapidly from around 10% to near zero with increasing dose of implanted ions. The total moment of the films drops to zero after about four monolayers equivalent coverage of ions (about 2xc3x971016 ions/cm2).
MR heads having defined sensors of the types as described in the first and second embodiments may be incorporated into a disk drive according to a third embodiment of the present invention. The disk drive includes a magnetic recording medium, a MR head with a defined sensor, an actuator connected to the MR head for moving the MR head across the magnetic recording disk, and a mechanism for moving the disk relative to the MR head.
MR sensors and disk drives made according to the various embodiments of the present invention exhibit edge definitions with narrow track widths suitable for future high density magnetic recording products.